Gripping tools usually take the form of either a pair of pliers or of an adjustable wrench depending on the particular job for which they are intended to perform.
A problem typical of pliers is that after they have gripped an article, the exerted pressure on the article continues to increase with sometimes deleterious results. Furthermore the gripping force of the jaws varies as a function of the gripping pressure on the handles of the pliers by the operator. An advantage of the pliers is that they are very easy and fast to attain an open or closed configuration, by simply opening or closing the handles.
On the other hand, adjustable wrenches do not attain the open or closed configuration nearly as easily and fast as pliers, and they do not have the gripping power of pliers, but they lock in a series of positions, thus avoiding the deleterious effects of uncontrollable excessive pressure.
The advantages and disadvantages of these gripping tools result mainly from the fact that the jaws of the pliers operate through a rotational motion around a pivot point, while in the case of the adjustable wrenches, the jaws move in a linear sliding mode. Many attempts have been made so far to attain the advantages and eliminate the disadvantages of both mechanisms in a single tool without success.
One category of such attempts, represented for example by U.S. Pat. No. 4,542,669, involves the so-called "vise grip pliers", the mechanisms of which are based on the "over the center" principle. Although these tools are invaluable when used as portable vises, they lack fast operation as that encountered in common pliers, since their jaws have to be brought first to close proximity to the size of the article to be gripped, usually through a screw mechanism, and then brought to the final gripping position by energizing the "over the center mechanism" as a second operation. In addition, a separate lever has to be pressed at the end of the operation for releasing the gripping force.
To avoid the necessity of the initial step, Ellison (U.S. Pat. No. 3,261,243) uses a rachet/toggle clamp, which engages as an "over the center position is approached. However, an additional separate lever has to be pressed again for releasing the gripping force.
Another type of adjustable wrenches, which however, also lack the advantages of pliers, since among other problems both hands of the operator have to be employed for their use, are described for example in U.S. Pat. Nos. 681,487 and 1,408,524. Both employ engaging means in the form of two flat sets of teeth which engage each other to prevent further movement of the jaws after an article has been gripped thereinbetween.
In U.S. Pat. No. 1,528,247 there is shown a pair of pliers including a pin, which can be rotated to lock the jaws of the pliers in a limited number of predetermined positions.
A somewhat improved version of this type of locking devices is shown in U.S. Pat. No. 1,271,911, where a pin is automatically inserted into a rachet mechanism during the gripping operation of the jaws. However, this mechanism also lacks the continuum of locking positions needed for making this tool actually useful and practical. For one pin to counteract successfully the huge forces acting on it, it has to have sizeable dimensions, thus considerably limiting the number of locking positions.
Although a gripping tool disclosed in U.S. Pat. No. 4,890,520 is a considerably improved version of the type of tools described above, regarding easy and fast operation, its lock-release mechanism requires high accuracy, and therefore it may not be easily and inexpensively mass-produced.